We have a bunch of those cheapy little digital kitchen timers where I work. Every morning for a while I’d set a couple so they wouldn’t go off until after I left, then hide them. Under the floortiles, behind the maps on the walls, taped underneath chairs. It took a week before the dayshift supervisor finally cracked and sent out a hysterical email demanding the head of the person responsible.
One that is really harmless and stupid that I like to do is go to a friend’s computer while they’re away from their computer and send myself an email. Usually something about how stupid they are or how attractive I am. Then I go back to my desk and reply to the message, feigning surprise that they finally got down the ultimate truth of the matter.
One that my father and his colleagues pulled on one of their work buddies: The person in question had bought a new fuel efficient car and was bragging to all his friends how much he would save on gas vs. their gas guzzlers. His co-workers all pitched in and refilled his tank every day. After a couple of weeks the victim stopped bragging about his stupendous gas mileage and returned to the dealer to ask what was wrong with his car. This was revealed when, back at the office, he demanded to know which of his friends was playing tricks on him (which was the service deparment’s guess when he reported his weird “problem” with his new car.)
You could put Kool Aid powder in a gelatin capsule. The hot water would melt the capsule and hilarity might ensue.
Just put food coloring on the bullion cube… red is very nice… or green… and use lots… 
Go to youtube.com and search on pranks, or just click on
to check out the Japanese Portapotty prank. The pranks toward the end - at lakeside venue - might make you laugh.
A bit more elaborate (and I’ve posted this in other prank threads, but whatthe hell).
I had a co-worker who, like clockwork, would go take crap at 10:00 am (we figured this out by the newspaper under his arm).
One time I gave him 2 minutes head start and followed him into the bathroom.
No one else was there. I had brought with me a cup of water. I went into the adjoining stall and stood there like I was going to urinate (unzipped to make it sound authentic). I proceeded to poor the water into the bowl, like I was peeing and slowly started to move the stream closer to his stall until I was missing the bowl entirely and spilling it on the tiles, splashing his shoes. He yelled out some expletives but then started laughing when he recognized my shoes (which were a rather unique boot style).
Do you have a copy of this notice in word document?
Two words: Rit Dye
That, my friend, is happening tomorrow at work!
When I was in the Air Force in Korea, we didn’t have individual desks but rather one big community desk. The rule was to hang up your coffee cup (on the hooks behind the coffee bar) at the end of your day. We were a 24/7 workcenter. If you forgot, the next shift would fill your cup up with shredder shavings and water and put it in the freezer.
Once, a guy left his cup on the desk on his Friday. After freezing the cup, we’d go in all weekend and poor a little more water on it. When he came back to work, his cup was embedded in a column of ice in the middle of the freezer. It was all fun and games until he poked a hole in the freezer with a screwdriver trying to get it out.
I did a work placement at an engineering consultancy a few years back. I missed out on the previous year’s work placement prank, a student took a powerful adhesive the company had a sample of and stuck a coin to the footpath opposite the offices.
Not only did they have a great view of people stooping and trying their hardest to pick up the coin, but the main BBC offices in Northern Ireland were just across the road. After failing to life the coin, the victims of the prank started to wave at the BBC building, just to let them know they thought it a good joke 
We have a lunch cook in the hotel I work in who likes to steal our sixth pans and third pans when no one is in our kitchen (we are only open for dinner). We got the idea to fill them with water, and place them on a high shelf, so when he sneaked upstairs to steal the pans, he got soaked.
There is also the timeless kitchen gag of rigging the cooler doors with a pan full of water… soggy socks… heheh
Switching keys on the keyboard of a hunt-and-peck type person is always fun. Go for M and N - it’ll take a while to notice.
If you don’t have optical mice, take out the ball. If you do have optical ones, put a little bit of tape over the laser.
Change sounds around. For example, if I wanted to be very cruel, I would copy the annoying siren “virus found” alert from my virus checker onto a USB memory stick, pop it onto the targets computer, and set it as every sound - asterisk, minimize/maximize, open program, etc.
I’m so glad I don’t work with any of you bastards.
This is much more of a “revenge” inspired prank I read of once: gain access to someone’s windowless office or the conference room. Unscrew the plates around several electrical outlets on opposite walls. Stuff raw chicken inside the cavity.
Now I’m guessing you-all are glad you don’t work with THIS bastard.
TheLoadedDog…
You knocked me out with your pranks (plus the refinements)!!!
Thanks!
I just puked a little in my mouth just thinking of this.
Try this, Kalhoun…
When I was in high school and working part time at an A&P, I was in the stock room where I stuck my hand beneath a small pile of cardboard and it ended smack dab in the middle of a host of roaches. Yuck!!!
I’m so going to Hell for making this easy:
You know those ‘drink packets’ that you pour into water bottles to get different flavors? (grape/rasberry/cherry/lemonade/ etc) Sounds like 1-2 packets would do nicely.
Brand names I can think of: I know that Crystal Light makes them and Wylers, but the box on my desk is '4C TotallyLight “Tea2Go” '. One box contains 20 individual serving packets that fit easily in jeans or jacket pockets. The best part is that no one will ever ask what you’re doing with one.
Note: You might need to carry a Leatherman to unscrew the shower head on the fly, but very few people will say ‘no’ if you tell them you need to use their bathroom.
Now, about that NOC list…
Already mentioned, but I’ve done the “desktop screenshot as wallpaper” myself. Good fun as the target finds all his apps “locked up”, prompting them to reboot their machine only to find all their programs still open and frozen.
Another time I worked in a computer lab where I would periodically remote login to my coworker’s computer in the cube next door and eject the CD tray. After a week or so of this I commented to him that sometimes the CD tray on my computer would just pop open by itself and he was like “yeah, that happens to me, too.” He never figured out what was going on.
At the same company, our computers had mechanical mice. Every single time a certain coworker got up from his desk, I’d put a piece of tape over the mouse ball. After a couple days, he got into the habit of checking the mouse whenever he got back, so I started opening the mouse and putting a bit of tape over one of the wheels inside. His mouse would look untampered and moving it would stop the screensaver, but the pointer would only move along one axis.
Not sure if this fits, but I also have a screensaver I wrote called the “TURT-L Geosproccessing Toolkit” that looks like some kind of geospatial batch process is running. It’s got windows that open up with progress bars and option buttons and status windows with warning messages. It’s all realistic-looking, but total gibberish and does absolutely nothing. The tipoff is that if you keep an eye on the status window, you’ll occasionally see football penalties or sports scores go by. At the place I worked for at the time we all had Sun workstations on our desk, but there were four common PC’s in one corner. Kept people from jumping on the PC I was using because they think I have a job running.
Good times.